Glass itself is a material less subject to alteration. However, it is known that the surfaces of glass sheets stored for a long period of time undergo alteration called “weathering”. The weathering occurs due to the fact that adhesion of water contained in the environment to the glass surface causes gradual dissolution of the components of the glass and thereby leads to a change of the state of the glass surface. The components dissolved from the glass are alkali metal ions as typified by sodium ions. With the dissolution of the alkali metal ions, the pH of the water adhering to the glass surface increases and reaches an alkaline pH. The silicon-oxygen (Si—O) bonds forming the framework of the glass is susceptible to erosion by alkaline solutions. On the glass surface with advanced erosion, insoluble matters may precipitate and thereby cause the appearance of white tarnish called “dimming”. The main one of the insoluble matters is sodium carbonate produced by reaction between carbon dioxide in the air and the dissolved sodium ions. In addition, the glass surface may look rainbow-colored because of the presence of an altered surface layer having a reduced refractive index due to the dissolution of sodium. This alteration is called “staining”.
Patent Literature 1 proposes applying an organic acid to a glass surface in order to prevent weathering. Adipic acid, fumaric acid, etc. are mentioned as examples of the organic acid (left upper column of page 2 in Patent Literature 1). The organic acid inhibits the erosion of the glass surface caused by alkaline solutions and prevents the occurrence of weathering.